1. Field of the Invention
The present invention relates to an acceleration control apparatus and, more particularly, to an acceleration control apparatus for controlling acceleration by directly comparing an acceleration command value and an actual value of acceleration.
2. Description of the Prior Art
In the field of tool feed control of machine tools, acceleration control has been performed in order to reduce its mechanical impact and to enable high-precision positioning.
A conventional type of acceleration control apparatus for performing such acceleration control is arranged as shown in FIG. 8. In FIG. 8, the illustrated acceleration control apparatus includes the acceleration commanding unit 10, a current command computing circuit 2 for calculating a current command value on the basis of the output signal of the acceleration commanding unit 10 and that of the position detector 61 directly connected to the motor 50, a driving control circuit 40 which forms a current minor loop, and a motor 50 to be controlled. The current command computing circuit 2 converts an acceleration command value .alpha.c into a current command equivalent signal of an amplitude proportional to the acceleration command value .alpha.c output from the acceleration commanding unit 10 (in FIG. 8, K.alpha.I represents the acceleration/current amplitude conversion constant). Accordingly, the acceleration of the motor 50 is controlled on the basis of the acceleration command value .alpha.c and a position signal (angle of rotation).theta. of the rotor of the motor 50. In other words, such a conventional acceleration control apparatus is not designed to directly form an acceleration loop for controlling the acceleration of the motor. Therefore the acceleration command value .alpha.c is converted into a corresponding current command, and a current minor loop provided in the driving control circuit 40 is substituted for the acceleration loop. This primarily derives from a historical background.
The aforesaid conventional acceleration control apparatus, however, involves the following problems.
In general, the substitute current minor loop is incorporated into a speed control loop. Therefore, in any of the cases where a load varies owing to friction, where the correlation between an assumed current amplitude and the torque generated by the motor 50 (a so-called torque constant) differs from an actual torque constant, and where the inertia of a load connected to the motor 50 differs from an actual inertia, it is impossible to exert accurate control based on the acceleration command value .alpha.c, and this causes a significant error between the assumed and actual acceleration. When such an error occurs, the error of a gain (cutoff frequency) of the aforesaid speed loop results. In addition, in a case where the gain of the speed loop is reduced owing to the error, an overshoot or vibration may result. Accordingly, it is pointed out that the current minor loop is not suitable for use with precise machine tools.
Secondly, since the substitute current minor loop is typically constituted by an analog circuit, an offset and a drift are likely to occur, and this may induce variations in the torque of the motor 50. For instance, in a case where the current minor loop is applied to the control of the tool feed shaft of a machine tool, a fringe pattern may be formed on a worked surface since it is impossible to fix its feed speed.
This problem is significantly important in terms of the installation environment of machine tools or various other circumstances which surround users.
To overcome such a problem, it is considered that the current minor loop is constructed by digital processing means. However, a high-speed and high-precision A/D converter is needed in order that the entire control system may be constructed as shown in FIG. 8 while the current command value and the feedback current value are digitized. However, this idea is not practical for economic reasons. In addition, since an offset and a drift are involved in such an A/D converter, this is not an essential countermeasure. Moreover, arithmetic operations upon the current minor loop gain must be performed by a CPU which stores software therein, with the result that an enormous throughput is needed because the motor 50, for example, a three-phase a.c. motor requires compensation elements for each of the three phases. Accordingly, the duration of a sampling time must be enlengthened or a high-speed CPU must be used. However, in the former case, current response deteriorates and controllability is thus lowered since the current loop gain lowers. The latter proposal is difficult to carry out for economical reasons similar to those mentioned above in connection with the A/D converter.